1. Field of the Invention
The present invention relates to a sheet transfer device, and more particularly to a sheet transfer device capable of preventing diagonal advancement of sheet.
2. Description of the Prior Art
Printing apparatus, particularly sheet-fed printing apparatus, requires a secure transfer operation, one by one, of sheet materials previously cut to a predetermined size. This requirement is particularly emphasized in case said printing apparatus is for example a computor terminal consuming a large quantity of sheet materials.
The commonly encountered troubles in the sheet transfer are represented by diagonal sheet transfer wherein the sheet is advanced in a diagonal direction and superposed transfer wherein plural sheets are advanced simultaneously, and the prevention of the former is essential for improving the reliability of the apparatus as it frequently gives rise to sheet jamming in the recording mechansim.
The one-by-one transfer, from the upper part of sheets stacked in a storage device, is usually achieved by a pair of transfer rollers which should freely follow the unevenness or inclination of the sheet surface thereby constantly applying the same pressure for both rollers and thus preventing the formation of undesirable fractional force not directed in the proper direction of transfer, which would otherwise give rise to a diagonal transfer of the sheet.
In ordinary electronic copying machines, the sheet materials, for example recording sheets, are accommodated in a relatively limited quantity in a cassette which is structured to support the stacked sheets by means of a spring, etc., and to uniformly press said sheets against the transfer rollers.
In such conventional apparatus, the uniform mutual contact between the recording sheet and the transfer rollers has thus been realized by uniformly elevating the stacked sheets rather than by providing the transfer rollers with a non-uniform pressurizing mechanism.
However, in a recording apparatus consuming a large quantity of sheet materials within a limited time (for example a recording apparatus disclosed in the U.S. patent application Ser. No. 616,675, now U.S. Pat. No. 4,059,833, of the present applicant wherein the information is recorded on a photosensitive drum by means of a laser beam modulated by the output of a computer and then transferred onto a recording sheet), it is preferable to accommodate an elevated quantity of recording sheets, for example 3,000 sheets, in the storage device in order to extend the interval between the sheet replenishments. In such case with such bulk quantity of stack, it is desirable that the transfer rollers are capable of following the stacked sheets, thus providing non-uniform pressure thereto.
For the purpose of one-by-one transfer of sheet materials from such storage device, there is already known a sheet transfer device as illustrated in FIG. 1, wherein the sheets 10 stacked in a sheet storage device (not shown) are advanced in the direction of arrow F by means of rollers to be explained later.
There is provided a drive shaft 11 arranged approximately parallel to the front edge of said sheet 10 and driven by a paper feed motor (not shown). On said drive shaft provided rotatably are arms 12-1, 12-2 which support rotatably and with a certain play a roller shaft 13 which in turn supports rollers 14-1, 14-2 so as to be rotatable only in one direction and also supports a gear 15 firmly mounted on said roller shaft and linked through a chain 17 with a gear 16 mounted on said drive shaft 11 thereby rotating said rollers 14-1, 14-2 by driving said drive shaft 11.
There is also provided a shaft 18 for connecting and fixing said arms 12-1, 12-2 and for supporting weights 19 for adjusting the pressure of said rollers applied on the stacked sheets 10.
In such sheet transfer device, the arm 12-1 can be twisted with respect to the arm 12-2 due to the presence of a certain play in the mounting between the arms 12 and the shaft 18 or roller shaft 13, and, as shown in FIG. 2, the roller shaft 13 is capable of assuming a position inclined by an angle .theta. when the sheet 10 becomes inclined with respect to the drive shaft 11, thus enabling intimate contact of the rollers 14-1, 14-2 with the sheet 10.
In such device, however, it was experimentally confirmed that, even if the rollers 14 in the stationary state thereof apply the same pressures to the sheet 10, the transmission of a rotary drive force to the drive gear 15 causes the roller 14-1 located closer to said gear 15 to sink and the other roller 14-2 to rise, thereby rendering the drive force of the roller 14-1 larger than that of roller 14-2 and thus creating a tendency of diagonal advancement of the sheet 10.